A series of mutants of E. coli in which total fatty acid synthesis has been altered is now available. The fatty acids contained in the phospholipids of both the inner and outer membranes of the mutant forms can be dramatically altered. This is done by supplying either saturated or unsaturated fatty acids to the cells through the growth media. Low angle x-ray diffraction will be used to obtain structural data such as the bilayer spacings, fatty acyl packing spacings, and temperature regions at which the phase transitions occur. The structural parameters will first be determined for inner and outer membranes from wild type cells. Since the outer membranes contain a great deal of lipopolysaccharide, notable differences may be detected between the inner and outer membranes. Next, for both membranes, the relationship between fatty acyl chain length and bilayer spacings will be determined. This will be done both above and below the temperature mediated phase transitions. Combining this structural information with data from other investigations, an attempt will be made to correlate the overall structural information with the physiological functions of the membranes.